What is the thrust force on an 8 in. (205 mm) ductile iron 90° bend with a water test pressure of 200 psi (13.8 bar)?

The thrust force on a pipe bend is a result of the change in momentum of the water flow and the static pressure acting on the internal area. The calculation is most reliably performed using the reference data provided in the annex of the relevant NFPA standard. According to NFPA 24, Table A.10.8.2, the thrust force for an 8-inch, 90° bend at a pressure of 100 psi is 6,843 lb. Since the thrust force is directly proportional to the pressure, the force at the specified test pressure of 200 psi is calculated by doubling the table value: 6,843 lb × 2 = 13,686 lb. This value corresponds directly to option C.

A. This value is significantly underestimated and does not align with the thrust force generated under the specified pressure and pipe diameter.

B. This value incorrectly approximates the force as simply Pressure × Area (P×A), failing to account for the vector change in direction at the 90° bend.

D. This value is a significant overestimation of the thrust force and is not supported by standard engineering calculations or reference tables.

1. NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances, 2022 Edition. Annex A, Section A.10.8.2, Table A.10.8.2 "Thrust at Fittings at 100 psi (6.9 bar) Water Pressure". The table explicitly lists 6843 lb for an 8 in. pipe with a 90° bend.

2. Ductile Iron Pipe Research Association (DIPRA). Thrust Restraint Design for Ductile Iron Pipe, 13th Edition. Page 5, Section "Thrust Force Computations". The document provides the standard formula for calculating thrust force: T = 2PA sin(θ/2), which validates the values found in the NFPA 24 tables.